1. Field of the Invention
This application relates to a new and novel process for recovering alcohols from aqueous strong acid streams. More particularly, the present application describes a process for the separation of alcohols from aqueous acid solution by permeation of the alcohol through a selectively permeable membrane comprising an organic acid-modified polymer.
2. Description of the Prior Art
The large-scale manufacture of alcohols from olefins is of considerable importance both for the alcohol produced and as a pathway in other processes. Isopropyl alcohol (IPA), for example, which is manufactured from propylene, is used as an ethanol denaturant and a solvent as well as in the production of acetone by catalytic dehydrogenation. Sec-butyl alcohol (SBA), obtained from butylenes, is used predominantly in the production of methyl-ethyl-ketone (MEK) by dehydrogenation.
The conventional method of obtaining alcohol from the corresponding olefin is by absorption of gaseous olefin (or "extraction" of liquid olefin) (the term "absorption" will be understood to refer hereinafter to both processes) in an aqueous solution of strong acid, typically sulfuric acid. This process comprises two steps: sulfuric acid-catalyzed esterification of the olefin to give a stream identified as sulfuric acid extract (SAE) which comprises the mono-and di-alkyl esters of sulfuric acid corresponding to the olefin used, some alcohol, sulfuric acid, hydrocarbon by-product and unreacted olefin; and hydrolysis of the sulfated ester to give alcohol and sulfuric acid.
For example, the absorption of butene in the membrane with sulfuric acid to form sec-butanol and the sec-butyl ester of sulfuric acid can be illustrated by the following equation: ##STR1## Thereafter, water is admixed with the SAE as it is withdrawn from the absorber in order to hydrolyze the ester and to facilitate alcohol recovery by steam stripping. There is thereby produced a diluted sulfuric acid stream which must for economic reasons be treated to concentrate it with respect to its sulfuric acid content, after which it is recycled to the olefin absorption step.
While it is also known to obtain alcohols by means of direct catalytic hydration, this process has the disadvantage of being equilibrium constrained, thus requiring olefin feeds of high purity.
Of course, other alcohols may be produced by absorption of olefins in acid, generally comprising saturated mono-alcohols having from 2 to 8 carbon atoms per molecule, and preferably having 3 or 4 carbon atoms per molecule. Examples of such alcohols are ethanol, iso-propanol, iso-butanol, sec-butanol, the pentanol isomers, etc., preferably the propanol and butanol isomers, most preferably isopropyl alcohol and sec-butyl alcohol.
Steam stripping the SBA and reconcentrating the spent sulfuric acid by distillation are both energy intensive processing steps. For example, there is an energy toll of about 1 to 2 lbs. steam/lb. alcohol product obtained in the steam stripping of the sulfuric acid extract, about 1 to 2 lbs. steam/lb. alcohol obtained, for reconcentrating the acid; and about 2 to 3 lbs. steam/lb. alcohol product obtained for, e.g., SBA distillation. Therefore, it will be apparent that means for recovering the alcohol product from the sulfuric acid stream at reduced energy cost would constitute a significant improvement over conventional practices in the manufacture of alcohols by absorption of olefins in acid.
Further, many lower molecular weight alcohols are totally miscible with and form azeotropes with water. Azeotropes at the azeotropic point give vapor of the same composition as the azeotropic liquid and thus cannot be further concentrated by normal distillation no matter how efficient the fractionating column used. Thus an alternative means to effect separation of such mixtures is highly desirable.
Various means have been suggested for improving the efficiency of such a process. U.S. Pat. No. 4,538,010, for example, describes an improved process for recovery of alcohols from the concentrated aqueous strong acid solution co-produced in their synthesis by acid absorption of olefins, the improvement residing in the use of a carboxylic acid extraction solvent to recover the alcohol from the strong acid extract, the resulting carboxylic acid extract phase being substantially free of water or strong acid. A heavy phase comprising substantially reconcentrated strong acid solution containing alkyl moieties is thereby also formed, which is suitable for recycle directly to the absorber. While the energy costs associated with acid reconcentration are thereby reduced relative to conventional processes, the large volumes of carboxylic acid extract required in the process introduce difficulties in handling as well as the added expense of the extraction solvent itself.
It is known in the art that certain membranes are permeable to molecules containing hydroxyl groups, such as water and aliphatic alcohols, and that certain of these membranes selectively permeate water over alcohols from solution containing the two. For example, U.S. Pat. No. 3,950,247 and 4,199,445 (the latter having issued on a divisional application based on the '247 patent), disclose a process for dehydrating aqueous solutions containing soluble organic or inorganic compounds by contacting the mixture against one side of an organic polymer membrane of polyvinyl chloride or having active anionic groups derived from strong acids, and withdrawing at the second side a mixture in the vapor phase having increased water concentration relative to the feed. Notably, in Example 1, a copolymer of styrene and acrylic acid is used to concentrate a formalin solution containing about 37% formaldehyde, 53% water, 0.05% formic acid (pKa=3.75), and 10% methanol, by selectively permeating water along with the formic acid. Thus, it is taught to use an organic polymer membrane to remove acid and water from a solution also containing alcohol and formaldehyde. In Example 7, where a sulfonated ethylene membrane was used to dewater a three-component system containing water, methanol and formaldehyde, but not acid, the order of selectivity was determined to be water&gt;methanol&gt;formaldehyde. Finally, Example 18 teaches dewatering of alcohol solutions, including azeotropic mixtures, by preferential permeation of water through certain organic polymer membranes.
It is further known that certain perfluorinated ionomer membranes with pendant sulfonate groups in the hydrogen or cationated form are permeable to molecules containing hydroxyl groups, such as water and aliphatic alcohols. In Examples 14, 15 and 16 of U.S. Pat. No. 4,199,445, nitric acid solution is concentrated by permeation of water through polymer membranes containing sulfonic acid groups, including the XR membrane of DuPont, which is a sulfonated perfluorinated polymer. Cares, U.S. Pat. No. 4,065,512, teaches dehydration of t-butanol by contacting with a perfluorosulfonate acid resin while passing dry fluid on the other side of the membrane, thereby removing the water of dehydration through the membrane. Cabasso et al. describe the separation under pervaporation conditions of alcohol/water vapor mixtures by Nafion 811 hollow fiber membranes, the water preferentially permeating through the membrane (I. Cabasso et al., "The Permselectivity of Ion-Exchange Membranes for Non-Electrolyte Liquid Mixtures. I. Separation of Alcohol/Water Mixtures With Nafion Hollow Fibers," J. Membrane Sci. 24, 101-119, 1985). The permeability of perfluorinated ionomer membranes has also been used to advantage to separate water vapor from hydrocarbons, M. L. Langhorst, "A Hollow Fiber Device for Separating Water Vapor from Organic Vapors", Am. Ind. Hyg. Assoc. J., 44, 592, March, 1983, and alcohols from hydrocarbons, I. Cabasso, "Organic Liquid Mixture Separation by Permselective Polymer Membranes. 1. Selective and Characteristics of Dense Isotropic Membranes Employed in the Pervaporation Process," Ind. Eng. Chem. Prod. Res. Dev., 22, #2, 313 (1983). In Vaughan, U.S. Pat. No. 4,532,347, oxygenated hydrocarbons such as alcohols are removed from fluid mixtures by permeation through a perfluorinated membrane with an extracting solvent containing a reactant which by reacting with the hydrocarbons maintains a high concentration gradient of the hydrocarbon across the membrane.